The present invention relates to a method for melting glass, more precisely, to a method for melting glass to diminish or extinguish a foam layer formed on molten glass in a glass melting furnace during preparation of glass.
A glass melting furnace for continuously producing various glass products such as flat glass, container glass, CRT, glass tubes, etc. is basically composed of a melting chamber, a refining chamber and a heat recovering apparatus. The above chambers are divided with a partition called a shadow wall, a neck, a throat, etc.
In an example of a side port type furnace for melting glass, glass raw material is continuously fed from a raw material port provided at the upstream end of a melting furnace. The glass raw material is melted by means of an air burner or an oxygen burner using a fuel such as heavy oil and natural gas, arranged at the both sides of the melting furnace. After the raw material has been fully melted, molten glass is adequately refined and then is taken out from the downstream end of a refining chamber, and it is formed into glass products having desired shapes. In this context, an air burner means a burner using air as an oxygen source for combustion, and an oxygen burner means a burner using air rich in oxygen or a pure oxygen gas as an oxygen source for combustion.
During a procedure of glass melting, glass raw material fed from the material port is composed of cullet glass and a glass raw batch comprising various ingredients in order to prepare glass products having desired compositions. In general, the above glass raw batch and cullet glass mixed in a predetermined ratio are together supplied from the material port into a melting chamber. The fed material forms a raw material layer floating in molten glass and is pushed by the raw material newly fed towards a central portion of the melting chamber, as successively melted.
In the glass melting furnace mentioned above wherein the surface of the raw material layer is heated by a heating equipment such as an oil burner and a gas burner, the unmelted glass raw material layer is melted from its surface and gradually diminishes as it proceeds from the raw material port to the central portion. Then, it is extinguished at a position where the fed amount is balanced with its melting velocity. Around the area where the raw material layer extinguishes, foam is formed by a reaction of the material, and form a foam layer usually extending from a position at which the raw material layer extinguishes to a position of the highest temperature in the melting furnace to cover the surface of molten glass.
The above foam layer has a foamed surface which scatters thermal rays and reflects radiant heat from the flame of burners, a fume exhaust gas and refractory bricks. As a result, thermal transmittance to the molten glass beneath the foam layer is hindered to result in the lowering of the heat efficiency. The foam layer reflects radiant heat toward the ceiling and the side walls of a furnace to raise their temperature, which is one of the cause of the damage of the refractory brick. Further, the foam layer is numerous accumulated foam including gas in their inside. If the foam is not removed during the refining procedure, they will impair the quality of the molten glass and lower the yield of the product due to the foam contained.
The foam layer on the molten glass becomes thicker and longer, and tends to cause more damage toward the production, when glass products are manufactured from the molten glass on a lager scale and at a higher production speed. There is also a tendency for an increase in a thickness of the foam layer, when an oxygen type combustion of oil or a gas using an oxygen burner is employed. Accordingly, the foam layer on the molten glass is required to be suppressed or extinguished in the case of an oxygen type combustion using an oxygen burner as well as an air type combustion using an air burner.
The object of the present invention is to provide a method for melting glass for solving various problems caused by the foam layer, whereby the formation of the foam layer can be suppressed, and thickening of the foam layer can be prevented, and the formed foam layer can be extinguished or diminished in a short period of time.
The present invention has been made in order to solve the problems mentioned above, and is to provide a method for melting glass, which comprises melting glass material fed to a glass furnace to prepare molten glass, wherein supplying at least one metal compound which is a compound of at least one metal selected from the group consisting of aluminum, titanium, silicon, zinc, magnesium, iron, chromium, cobalt, cerium and calcium is supplied to a foam layer formed on the molten glass to diminish or extinguish the foam layer.
In the present invention, the metal compound having a function of diminishing or extinguishing the foam layer formed on the molten glass is selected from at least one compound of at least one metal selected from the group consisting of aluminum, titanium, silicon, zinc, magnesium, iron, chromium, cobalt, cerium and calcium. One or at least two of the metal compound in the form of a solution, a suspension, a powder, or a gas is supplied the foam layer formed on the molten glass. When the above metal compound is supplied, the foam layer is immediately extinguished or diminished. However, the foam layer will be recovered to its former state, once the supply of the metal compound is stopped. Accordingly, it is preferable to supply the metal compound to the foam layer continuously or intermittently.
As a method of supplying the metal compound, it is possible to employ a method wherein the metal compound is directly supplied to the foam layer by a spray apparatus such as a spray nozzle, provided through the side wall of a furnace. In the case of a glass furnace wherein a gas or oil is burned using air, it may be practical to employ an indirect method, wherein the metal compound is supplied into air for combustion in glass furnace by means of a spray, and is carried with the air to a combustion area to reach the foam layer. Further, it is also possible to employ a method, wherein the metal compound is mixed in a heavy oil or a gas beforehand, and supplied to the foam layer while burning a fuel using a combustion burner.
Moreover, in the case of a glass furnace wherein a gas or oil is burned using an oxygen gas, it may be practical to employ an indirect method, wherein the metal compound is mixed into an oxygen gas for combustion in a glass furnace by means of a spray, and is carried with an oxygen gas to the combustion area to reach the foam layer. The above metal compound may also be sprayed to supply it to the combustion area in a glass furnace.
The metal compound employed in the present invention may be an inorganic compound or an organic compound. At the time of reaching the foam layer on molten glass, the form of the metal compound may be an unreacted state, a reaction intermediate compound or a reaction product.
In particular, at the time just before reaching the foam layer on molten glass, the form of the metal compound is preferable to be metal oxide particles as fine as possible, caused by an oxidation reaction of the metal compound with aid of high temperature in a glass furnace. This may improve dispersion of the metal compound into the foam layer and the foam layer tend be easily diminished or extinguished. Accordingly, it is particularly preferable to employ an organic metal compound which is easily to form metal oxide particles by a decomposition oxidation reaction at a high temperature.
As such a representative example of the organic metal compound, the following may be mentioned.
As an organic titanium compound, a titanium ester such as tetraethyl titanate, tetrabutyl titanate, tetraisopropyl titanate, tetraoctylene glycol titanate, or its derivative, a titanium chelate such as dihydroxytitan lactate, hydroxytitanium dilactate, or its derivative, a titanium acylate or its derivative, or a titanium oxalate may, for example, be employed.
As an organic silicon compound, tetramethyl silicate, tetraethyl silicate, or tetra-n-propyl silicate, may, for example, be employed. As an organic aluminum compound, acetylacetone may, for example, be employed.
Above such a compound may be used in the form of a solution having dissolved in water and/or in an organic solvent in an optional proportion.
Moreover, as such a metal compound, a chloride, a sulfate or a nitrate of titanium, silicon or aluminum, such as silicon tetrachloride, titanium tetrachloride, aluminum trichloride, aluminum sulfate and aluminum nitrate may, for example, be employed. Such a metal compound may be sprayed in the form of a solution.
A compound such as titanium tetrachloride(melting point: xe2x88x9223xc2x0 C., boiling point: 136.4xc2x0 C.) and silicon tetrachloride (melting point: xe2x88x9270xc2x0 C., boiling point: 57.6xc2x0 C.) is a liquid at room temperature, but it has a low boiling point and can be easily vaporized. Such compound may be advantageous in handling since it can be vaporized by heating, and can be supplied to a foam layer by means of a carrier gas without using a solvent.
As such a organic solvent for the organic metal compound mentioned before, a solution which can dissolve or disperse uniformly such organic metal compound may be employed. The following various organic solvents may be mentioned as an example. An alcohol such as methanol, ethanol and isopropyl alcohol, a hydrocarbon such as hexane, benzene, toluene and xylene, a hydrocarbon oil such as gasoline and kerosine, an ester such as ethyl acetate and isobutyl acetate, a cellosolve such as methyl cellosolve and ethyl cellosolve. Ethanol, isopropyl alcohol, toluene, ethyl acetate, kerosine, etc. may be mentioned as a particularly preferred solvent in view of their price, availability and handling.
It is possible to employ a powder of at least one metal oxide which may be selected from the group consisting of aluminum oxide, titanium oxide, silicon oxide, zinc oxide, calcium oxide, magnesium oxide, iron oxide, chromium oxide, cobalt oxide and cerium oxide. Such a powder may be carried as it is by means of a gas or may be suspended in water and/or an organic solvent to form a slurry, which may be sprayed by a spray nozzle.
As the reason why the supply of the above metal compound to the foam layer can diminish or extinguishment of the foam layer in the present invention, it is considered that the surface tension will be influenced by the use of the metal compound. That is, such a metal compound having affinity with the glass forming foam will deposit on the surface of the foam in the foam layer, and then will penetrate into the glass forming foam to weaken the bonding power maintaining foam, which will bring about breakage of the foam.
Now, the present invention will be described with a reference to a preferred embodiment of a glass furnace for operating the present invention as shown in the attached drawings.